How to Add DLLs to WSP Solution File

If you need to include additional dlls in your wsp file, then you should add entries to both your cab.ddf and your manifest.xml files. The entries in the cab.ddf indicate the location of the dll that needs to be copied, and in what folder to copy it to in the wsp. The manifest.xml entries indicate where the dll is located in the wsp, and where it how it should be deployed upon solution deployment.

Full code example on my blog: http://blogs.windwardreports.com/tomasr/2009/09/how-to-add-dlls-to-a-wsp-sharepoint-solution.html